Who does the maths and with which formulas

Who runs this website

KredytPartner is run by PIOTR LITWA Data & Software - a sole-trader business from Gliwice working in software and data processing. The content and the calculation engine are the responsibility of Piotr Litwa, a software engineer and the author of all the calculators and guides on the site.

Where the numbers come from

All the calculators on the site use one shared calculation engine. There is no separate code "for the payment page" and separate code "for the APR page" - so the same situation gives the same result no matter which page you reach it through. Here are the foundations each of them rests on:

Fixed (annuity) instalments

The standard annuity formula - the same one implemented in Excel by the PMT function. The monthly rate is the annual rate divided by 12, in line with the practice of Polish banks (not the effective rate). We build the schedule instalment by instalment: interest is charged on the current balance, and the rest of the payment goes towards the capital.

Declining instalments

The capital part is constant and equal to the loan amount divided by the number of instalments. We calculate interest on the falling balance, so the payment drops from month to month.

APR (RRSO)

We calculate the APR - in Polish RRSO - in line with the definition in the Polish Consumer Credit Act of 12 May 2011 (ustawa o kredycie konsumenckim): it is the annual rate at which the discounted sum of payouts equals the discounted sum of repayments. Mathematically, it is the internal rate of return (IRR) of the cash flows. Because such an equation cannot be solved with a formula, we find the root by bisection with a precision better than 0.0001 percentage points.

The cash flows include all the costs borne by the borrower: a fee deducted from the payout, insurance, monthly charges. That is why the APR in our calculator is sometimes higher than the one you see in an advert - the advert shows a "representative example", while you enter your own parameters.

Borrowing capacity

This is the only calculator on the site that is by nature an approximation - every bank has its own, undisclosed scoring model. Our model reproduces the logic common to the market and arising from Recommendation S of the Polish Financial Supervision Authority (KNF): we deduct living costs and existing commitments from income, and calculate borrowing capacity at an interest rate raised by a buffer for rate rises (KNF requires at least 2.5 percentage points). Treat the result as an order of magnitude, not a promise.

Compound interest, inflation, leasing

We calculate compound interest taking into account the compounding frequency and the timing of contributions (start or end of the period), optionally with the 19% Belka tax (Poland's capital gains tax on savings) and a conversion into today's purchasing power. We discount purchasing power using the compounded inflation rate. For leasing we separate the down payment, the financed part and the buyout value, because it is the buyout that decides whether a comparison with a loan makes sense. We calculate the tax shield for passenger cars with the cost limit in force from 2026 (PLN 100,000, PLN 150,000 or PLN 225,000 depending on CO2 emissions): we apply the limit to the car's value including non-deductible VAT and only to the capital part of the instalments. When comparing leasing offers, we work out the interest rate hidden in the offer numerically from the instalment, the down payment and the buyout.

How we check that it works

The calculation engine has a set of 75 unit tests, which we run before every release of changes. The key principle of these tests: the expected values come from sources independent of our code - from spreadsheet functions and from annuity tables. A test that compares code with itself checks nothing.

On top of that, every calculator shows on its page the formula it uses. You can take our result, enter the same numbers in Excel and check us. We encourage it - and we take it seriously: if you find an error in the calculations, write to us. We will fix it and thank you in the text.

How the guides are written

• Every number in the text is calculated, not made up. The numerical examples in the guides go through the same engine that powers the calculators. If an article says that a "0%" loan with a PLN 1,000 fee has an APR of 21.83% - you can check it in our APR calculator and you will get the same number.
• We also write about the uncomfortable things. In the guide on payday loans we show what rolling over a loan really costs. In the guide on overpayment - when overpaying does not pay off. A website that never advises against anything is not a guide, it is an advert.
• We give the date. Every guide shows its publication date and the date it was last updated. Regulations and interest rates change - you have the right to know how old the text you are reading is.
• We do not use artificial urgency. You will not find countdowns here like "offer expires in 3 minutes" or "only 2 places left". A borrowing decision for a dozen or more years should not be made under time pressure.

How we make money

Honestly: the site is free for you, but it is not free to run. We have two sources of income and both affect what you see - which is why we describe them openly.

Adverts. The site displays Google AdSense adverts. They are labelled and kept separate from the content. They have no effect on what we write in the guides or on the calculator results.

Affiliate commissions. When you take up an offer from a financial institution through us, the institution pays us a fee. This does not raise the cost of the loan for you - the commission comes from the institution's margin, not from your pocket. We mark affiliate links with the rel="sponsored" attribute.

Your data

The calculators run in your browser. The amounts, income and commitments you enter in a calculator are not sent to our server or saved anywhere. We collect personal data only when you fill in a contact or application form yourself - the rules are described in the privacy policy.

Company details and contact

A question about the methodology, a report of an error in the calculations or a partnership proposal - it all goes through the contact page. We reply within 2 working days.

The content on the site is educational and informational. It does not constitute investment advice, a financial recommendation or an offer within the meaning of Art. 66 §1 of the Polish Civil Code. The full terms of use are set out in the terms of use.